1. Field of the Invention
The present invention relates to a holographic laser and an optical pickup for use as the light source for reading out signals from optical disks such as compact disks (CDs), compact disks recordable (CD-Rs), digital versatile disks (DVDs), and digital versatile disks recordable (DVD-Rs), and which are compatible with a plurality of read out wavelengths.
2. Description of the Related Art
In the CD family of optical disks, signals are read or written using a semiconductor laser element having an emission wavelength of 780 nm. On the other hand, in the DVD family of optical disks, to improve the signal recording density, the signals are read or written using a semiconductor laser element having an emission wavelength of 630 to 690 nm.
When optical disks of the CD family and the DVD family are read out or written onto using the same optical disk apparatus, a plurality of semiconductor laser elements with different emission wavelengths are provided within that optical disk apparatus.
FIG. 13 is a structural diagram showing an example of a conventional optical pickup. This optical pickup includes in a single package for example a semiconductor laser 1, which emits light of two different wavelengths, an optical system for guiding read light from the semiconductor laser 1 to an optical disk M, and guiding light reflected from the optical disk M to a photodiode 5, and the photodiode 5, which is for receiving the reflected light and reading the signal.
The optical system includes for example a half mirror 10, which reflects read light from the semiconductor laser 1 and transmits light reflected from the optical disk M, a prism 11 for aligning the optical axes of the read lights of the two wavelengths, a collimating lens 12 for converging the read light, a total reflection mirror 13 for bending the optical axes, and an objective lens 14 for focusing read light onto the optical disk M and for converging light that is reflected from the optical disk M.
At the window of the semiconductor laser 1, there is a diffraction grating 2 for transforming one of the read lights of the two wavelengths into three beams.
FIG. 14 is a structural diagram showing another example of a conventional optical pickup. This optical pickup includes for example a holographic laser 3, which emits read light and receives light reflected from an optical disk M, and an optical system, which is for guiding read light from the holographic laser 3 to the optical disk M and guiding light reflected from the optical disk M to the holographic laser 3.
Integrated into the holographic laser 3 for example are a semiconductor laser, which emits read light of a single-wavelength, and a photodiode for reading out signals.
The optical system is made up of the collimating lens 12, the total reflection mirror 13, and the objective lens 14, for example.
With the configuration of FIG. 13, the number of optical components, such as the prism 11 for adjusting the optical axes, increases, because the read lights of two wavelengths are guided to the optical disk M and the light reflected from the optical disk M is returned to the single photodiode 5. As a result, the positions of the optical components have to be adjusted in more instances, which makes the adjustment during assembly complicated. Moreover, the optical pickup becomes larger, which complicates making the optical disk apparatus thinner and lighter.
With the configuration of FIG. 14, the optical pickup is limited to reading out only a single wavelength, and therefore is incapable of reading out different families of optical disks. Two optical pickups, each with the same configuration and which read different wavelengths, must be provided to be compatible with a plurality of read wavelengths.
When semiconductor lasers of two different wavelengths and a photodiode are integrated in the same holographic laser, light emitted from two different positions is returned to the light-receiving surface of the single photodiode, and therefore the dimensional precision and the assembly precision of the optical components must be extremely good.
The emission wavelength of semiconductor laser elements depends, for example, on the temperature and the intensity of the optical output, and therefore it must be taken into account that the plurality of read wavelengths fluctuate independently. The diffraction angle in holograms changes with the wavelength, and if the grating spacing is constant, then the shorter the wavelength, the smaller the angle of diffraction.
If there is only one type of optical disk to read and only a single semiconductor laser element, then the line at which the photodiode separates received light can be positioned in the direction in which the angle of diffraction of the hologram changes due to fluctuations in the wavelength, thus making it possible to obviate the effect of wavelength fluctuations.
However, when the optical disk apparatus is compatible with a plurality of optical disk types of different read wavelengths, then the fact that there are two semiconductor laser elements means that the two reflected laser lights are not necessarily both incident on the line at which the received light is separated, even if the line at which the photodiode separates received light is positioned in the direction that the angle of diffraction of the holograms changes due to fluctuations in the wavelength.
When semiconductor laser elements of two wavelengths having two light-emission points are used in the same chip, the emission points are close to each other, and therefore it is difficult to receive the light with the same light receiving element. This is because holographic elements have the characteristic that the angle of diffraction is determined by the wavelength when the same diffraction grating is used, and thus when the emission points are close to each other it is difficult to focus light with different angles of diffraction onto the same point. For example, when the distance between the laser light-emission point and the diffraction grating is approximately 2 to 3 mm and the distance between the diffraction grating and the light receiving surface of the light receiving element is approximately 1 mm, then, if the size of the laser light-emission point is approximately 150 to 250 μm, the light beams can be converged onto the same point, but when the size of the laser light-emission point is between several μm to several dozen μm, the diameter of the spot on the light receiving surface in turn becomes approximately 150 to 250 μm, and therefore the light cannot be focused onto the same light receiving element.
Furthermore, when the laser light-emission point is placed away from the optical axis of the optical system, aberrations occur each time light passes through a lens, thus negatively affecting optical pickup properties.